Wash the oranges in hot water and dab dry. Cut 2 oranges into thin slices (approx. 3 mm each). Heat 3 tbsp. grenadine in a large pan. Caramelise half of the orange slices for about 1 minute, turning once. Remove and let cool. Bring 3 tbsp grenadine to the boil again in the pan. Cook the rest of the orange slices in the same pan. Put 2 tbsp. grenadine in a springform pan (24 cm Ø). Place the orange slices slightly overlapping on the bottom of the mould, pushing the outer slices up 2-3 cm at the edges.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Finely grate the rest of the orange peel. Squeeze the orange. Mix flour, sugar, vanilla sugar, baking soda, baking powder and 1 pinch of salt. Mix soy yoghurt, oil, orange peel and 3 tbsp. orange juice. Stir into the flour mixture with the whisk of the mixer. Fill the dough into the form, smooth it down and bake in a hot oven for about 40 minutes.
Take out and let it cool in the mould for about 10 minutes. Then carefully remove from the edge of the mould. Carefully turn over onto a cake plate. Let it cool down. Serve with whipped cream, e.g. refined with a little grenadine.